1. Technical Field of the Invention
The present invention relates generally to an electronic control apparatus equipped with a timer-activated function which may be employed in automotive vehicles to perform a given task such as a fuel vapor leakage check upon activation of a timer during an off-state of an ignition switch, and more particularly to such an electronic control apparatus designed to monitor or diagnose a malfunction of a timer.
2. Background Art
Typical automotive engine control systems are designed to be activated by electric power supplied upon turning on of the ignition switch. However, in recent years, the need has arisen for initiating a given task at some specific time during an off-state of the ignition switch. Typical of such a task is a fuel vapor leakage check in an evaporative purge system (also called an evaporative emission control system) of automotive vehicles. The evaporative purge system is a system for avoiding the escape of fuel evaporative emissions or fuel vapors from a vehicle's fuel system to the atmosphere. For example, Japanese Patent First Publication No. 8-35452 discloses a typical evaporative purge system which works to adsorb fuel vapors in an adsorbent of a canister and siphon the fuel vapors from the canister into an intake pipe of the engine together with fresh air drawn from an air inlet of the canister to purge the canister in accordance with operating conditions of the engine.
If holes or cracks occur in the fuel tank or an evaporative emission path between the fuel tank and the canister, it may cause the fuel vapors to be released into the atmosphere without being adsorbed in the canister. In order to avoid air pollution arising from such a malfunction of the evaporative purge system, the fuel vapor leakage check is made to monitor a leak of the fuel vapors from the fuel system of the vehicle.
The fuel vapor leakage check may be achieved by keeping the evaporative purge system closed hermetically by a solenoid valve and measuring a variation in pressure within the evaporative purge system using a pressure sensor. A leakage check of this type may, however, have a difficulty in diagnosing the malfunction of the evaporative purge system correctly after a long period of high load engine running because of increased ease of evaporation of the fuel. This problem may be eliminated by activating a host microcomputer of an engine control ECU a preset period of time after the ignition switch is turned off to carry out the leakage check.
The above system requires a timer circuit such as a soak timer for measuring the preset period of time after the ignition switch is turned off. For example, Japanese Patent First Publication No. 2003-254172 teaches an engine control ECU with a soak timer. However, if any problem arises in the timer circuit, it will result in a difficulty in checking the fuel vapor leak, thus requiring accurate monitoring of an operating status of the timer circuit. Japanese Patent First Publication No. 2003-139874 discloses a monitoring system for timer circuits.
The operating status of the soak timer may be diagnosed by saving an activation history record indicating the fact that the host microcomputer has been activated after a lapse of a period of time measured by the soak timer in a standby RAM (i.e., SRAM) built in the host microcomputer, checking the presence or absence of the activation history record in the SRAM upon subsequent activation of the host microcomputer upon turning on of the ignition switch, and analyzing a count value of the soak timer, as sampled upon the subsequent activation of the host microcomputer.
The above method may, however, encounter a difficulty in writing the activation history record in the SRAM correctly upon activation of the host microcomputer by the soak timer if an operating voltage supplied to the host microcomputer has dropped below a lower limit required to write data in the SRAM correctly. This will result in an error in diagnosing a malfunction of the soak timer.